// Copyright (c) 2016 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

// An arena that consists of a single inlined block of |ArenaSize|. Useful to
// avoid repeated calls to malloc/new and to improve memory locality. DCHECK's
// if an allocation out of the arena ever fails in debug builds; falls back to
// heap allocation in release builds.

#ifndef NET_QUIC_QUIC_ONE_BLOCK_ARENA_H_
#define NET_QUIC_QUIC_ONE_BLOCK_ARENA_H_

#include "net/quic/quic_arena_scoped_ptr.h"
#include "net/quic/quic_flags.h"
#include "net/quic/quic_utils.h"

#define PREDICT_FALSE(x) x

namespace net {

template <uint32_t ArenaSize>
class QuicOneBlockArena {
    static const uint32_t kMaxAlign = 8;

public:
    QuicOneBlockArena();

    // Instantiates an object of type |T| with |args|. |args| are perfectly
    // forwarded to |T|'s constructor. The returned pointer's lifetime is
    // controlled by QuicArenaScopedPtr.
    template <typename T, typename... Args>
    QuicArenaScopedPtr<T> New(Args&&... args);

private:
    // Returns the size of |T| aligned up to |kMaxAlign|.
    template <typename T>
    static inline uint32_t AlignedSize()
    {
        return ((sizeof(T) + (kMaxAlign - 1)) / kMaxAlign) * kMaxAlign;
    }

    // Actual storage.
    // Subtle/annoying: the value '8' must be coded explicitly into the alignment
    // declaration for MSVC.
    QUIC_ALIGNED(8)
    char storage_[ArenaSize];
    // Current offset into the storage.
    uint32_t offset_;

    DISALLOW_COPY_AND_ASSIGN(QuicOneBlockArena);
};

template <uint32_t ArenaSize>
QuicOneBlockArena<ArenaSize>::QuicOneBlockArena()
    : offset_(0)
{
}

template <uint32_t ArenaSize>
template <typename T, typename... Args>
QuicArenaScopedPtr<T> QuicOneBlockArena<ArenaSize>::New(Args&&... args)
{
    DCHECK_LT(AlignedSize<T>(), ArenaSize)
        << "Object is too large for the arena.";
    static_assert(QUIC_ALIGN_OF(T) > 1,
        "Objects added to the arena must be at least 2B aligned.");
    if (PREDICT_FALSE(offset_ > ArenaSize - AlignedSize<T>())) {
        LOG(DFATAL) << "Ran out of space in QuicOneBlockArena at " << this
                    << ", max size was " << ArenaSize << ", failing request was "
                    << AlignedSize<T>() << ", end of arena was " << offset_;
        return QuicArenaScopedPtr<T>(new T(std::forward<Args>(args)...));
    }

    void* buf = &storage_[offset_];
    new (buf) T(std::forward<Args>(args)...);
    offset_ += AlignedSize<T>();
    return QuicArenaScopedPtr<T>(buf,
        QuicArenaScopedPtr<T>::ConstructFrom::kArena);
}

} // namespace net

#endif // NET_QUIC_QUIC_ONE_BLOCK_ARENA_H_
